The laws created by government typically do not allow you to
ride your bicycle on a superhighway. Likewise, Ethernet does not allow you to
send the end user data over the LAN. You must put the data in the proper
vehiclenamely, an Ethernet frame. After you put the data into a frame, you can
send it across the LAN.
The Ethernet header and trailer are simply additional
bytes of data that are used by the computers, NICs, and networking devices to
make the Ethernet work smoothly. For instance, the first 8 bytes in an Ethernet
header are called the preamble. The preamble contains
alternating 1s and 0s so that the NICs receiving the data know that a new frame
is being sent across the LAN.
The word frame happens to be a
particularly important term. Back in Chapter 3, "Building a Network: It All Starts with a
Plan," you read about networking standards and how the terms from the OSI model
describe networking standards and protocols today. The term "frame" refers to
the headers and trailers defined by any Layer 2 standard, as well as the data
inside the frame. The term "Ethernet frame" refers to a frame created for use on
an Ethernet, conforming to Ethernet protocol specifications.
As you will read in later chapters, protocols that match other
OSI layers also have headers, and the networking world uses names besides
"frame" to refer to those headers and data.
Throughout this chapter, I will cover more details about what
is inside the Ethernet frame header and trailer. To move forward, just think of
an Ethernet frame as the car that can actually be sent over the LAN, with the
end user data being the equivalent of whomever or whatever is in the car.
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